The invention relates generally to the construction of spring assemblies or the like. More particularly, it relates to the manufacture of strings of pocketed coil springs for use as the spring cores for mattresses, seat cushions or the like.
Mattress spring core construction over the years has been a continuously improving art with advancements in materials and machine technology. A well known form of spring core construction is known as a Marshall spring construction wherein metal coil springs are encapsulated in individual pockets of fabric and formed as elongate or continuous strings of pocketed coil springs. In an early form, these strings of coil springs were manufactured by folding an elongate piece of fabric in half lengthwise to form two plies of fabric and stitching transverse and longitudinal seams to join the plies of fabric to define pockets within which the springs were enveloped.
Recently, improvements in spring core constructions have involved the use of fabrics which are thermally or ultrasonically weldable to themselves. By using such welding techniques, these fabrics have been advantageously used to create strings of individually pocketed coil springs wherein transverse and longitudinal welds instead of stitching are used to form the pockets encapsulating the springs.
Once strings of pocketed springs are constructed, they may be assembled to form a spring core construction for a mattress, cushion or the like by a variety of methods. For example, multiple or continuous strings may be arranged in a row pattern corresponding to the desired size and shape of a mattress or the like and adjacent rows of strings may be interconnected by a variety of methods. The result is a unitary assembly of pocketed coil springs serving as a complete spring core assembly.
One improvement upon pocketed coil springs as described is a compound nested pocketed coil spring in which each pocket of a string includes two nested coil springs. In such designs, a first inner spring is typically shorter and smaller than a second outer spring. The first inner spring is nested within the second outer spring.
Spring core constructions employing compound nested pocketed springs provide the advantage of offering differing degrees of hardness to the spring unit. Varying degrees of hardness are usually achieved by varying the number of springs per unit area, commonly referred to as the "spring count" of the unit, or by changing the gauge of the wire from which the springs are manufactured. Compound nested pocketed spring coils are disclosed in PCT Application No. PCT/GB97/01759; U.S. Pat. Nos. 1,192,510; 2,567,520; 1,254,314; 882,654; and U.K. Patent No. 20,583. The inner and outer coil springs are nested so that the lower portion of the combined spring unit is reinforced by the inner spring making this portion of the unit much stronger than the upper portion. The upper portion may be flexible enough to provide a resilient and comfortable seating or sleeping surface and the lower portion strong enough to absorb abnormal stresses, weight concentrations or shocks without discomfort or damage.
Commonly, the inner spring of the nested compound spring unit is individually encased in a pocketed fabric material such as shown in U.S. Pat. No. 1,192,510, to minimize noise or interference during the flexing or compression of the compound spring unit.
Another advantage of such compound spring units when employed in a mattress or the like is that the inner spring of each compound nested spring unit is free floating or unsecured. As a result, when the mattress is inverted, the inner spring falls by gravity toward the lower face of the mattress. In this way, regardless of whether the mattress is inhibited or flipped, the inner spring is always at the bottom portion of the spring unit and the compound nested spring units provide a varying degree of flexure from the top to the bottom of the spring unit.
Even though spring units constructed from strings of pocketed compound nested coil springs as described provide many advantages, the manufacture and construction of strings of pocketed compound nested coil springs has proven to be very complicated and often problematic resulting in increased expense for such strings. The construction of strings of pocketed coil springs with a single spring in each pocket is well known in the art and, for example, disclosed in U.S. Pat. No. 4,439,977 which is hereby incorporated by reference in its entirety. The system disclosed in U.S. Pat. No. 4,439,977 includes a spring coiler which forms a coil spring and deposits it about the upper end of an arcuate delivery horn. As such, the formed coil spring is delivered by gravity in a generally vertical orientation for subsequent compression and insertion into the pocketing fabric material.
Another well known system for pocketing coil springs is commercially available from Spuhl AG in Switzerland. Examples of such machines include the Spuhl TF 90, 190 and 290 series machines. In such machines, a coiler forms a spring and deposits the spring into a trough in a generally horizontal orientation. The spring is then compressed horizontally by a compression paddle, rotated through 90.degree. and then while remaining compressed is inserted between the plies of a folded fabric which is subsequently formed into a pocket around the spring.
One technique for manufacturing pocketed compound nested coil springs which is compatible with the Spuhl-type machines is disclosed in UK Patent Application No. 9726333.9 which is hereby incorporated by reference. The system disclosed in that UK patent application calls for the outer coil spring to be pushed over the inner coil spring once it has been dropped into the trough in the generally horizontal orientation. Alternatively, the springs could also be nested by dropping the smaller inner coil spring into the trough in advance of a larger outer coil spring and pushing the inner coil spring into the outer coil spring while both are in generally a horizontal attitude.
While the system disclosed in UK Patent Application No. 9726333.9 provides opportunities to manufacture pocketed compound nested coil springs while utilizing the Spuhl-type coiling and pocketing machines, there is a need to provide alternative or additional systems which can be utilized on a production basis and lend themselves to further automation of the procedure so that the manufacture of pocketed compound nested coil springs may be as fully automated as the conventionally preformed production of single pocketed coil springs.